The present invention relates to a glass melting plant having a melting tank having end-fired heating, having side walls and a floor, and the melting tank having a melt surface of at least 40 m2. The melting plant is fed from one side, via a doghouse having a feeding device.
The melting tank standardly has an inlet for supplying the feed material charge, a preferably channel-shaped outlet for removing the molten glass, the inlet of the melting tank being connected to the doghouse and the doghouse having a roof that has an end wall facing the feeding device, the end wall enclosing, together with the roof, a gas compartment that is open toward the melting tank. The surface of a conventional doghouse is, as a rule, between 1.5 m2 and 3.5 m2. At the end situated opposite the inlet of the melting tank, the doghouse moreover has an opening in the end wall that is used to input the charge.
The glass melting process is an energy-intensive industrial manufacturing process. Therefore, a great deal of effort is made to reduce the energy required per ton of glass produced. The manufacturing of so-called soda-lime-silica glass provides an example of the high energy outlay. This group of a large number of glass compounds is used for 80% of glass production worldwide. It is the basis for the production of container glass and flat glass. Fossil fuels are predominantly used as the energy source for this process. These fuels are brought to exothermic reaction using either air or oxygen as an oxidant. The released energy is transferred to the melt bath, or to the raw material mixture (batch) placed onto the melt bath. In the melt process, a large portion of the energy is required to convert the raw material mixture into a liquid melt. This process is endothermic. Approximately 35% of the energy required for the melting and heating to the required process temperature is required for the chemical conversion. If the energy is added that is required to heat the mixture to the reaction temperature, this energy portion is over 60% of the total energy transferred to the melt bath.
For the continuous production of the glass melt in larger quantities, tank technology has predominated since the end of the 19th century. Here, the raw material mixture is continuously introduced in dosed fashion into a melting tank having a specified depth. Above the melting tank there is situated a combustion chamber in which fossil fuel is combusted with addition of an oxidant. Here the exhaust gas can be used to preheat the oxidant. The melting tank has a channel-shaped outlet or flue from which the completely melted and refined glass is supplied to manufacture.
Over time, the technical realization of glass melting plants has constantly been improved. This has had to do essentially with the demands made on quality, the lengthening of the lifespan of the melting plants, the reduction of investment costs, and the reduction of emissions. A large part of the investments also went towards reducing energy consumption. Calculated over its entire useful life, the energy costs of a glass melting plant are a multiple of the investment sum for the glass melting plant itself. Consequently, increasing energy costs provide, especially today, an essential economic argument for the significant efforts made to reduce energy consumption.
However, these developments quickly run up against technological limits. A theoretical boundary value can be defined as follows (see Conradt, “Comparative Analysis of the Performance of Industrial Glass Melting Furnaces,” in DGG GOMD Conference, Aachen 2014, Advances in Fusion and Processing of Glass).
Given a pure glass melt of a raw batch for a soda-lime-silica glass, and a standard exit temperature of the glass melt from the melting plant of 1200° C., a specific energy consumption of 2.1 GJ/t is to be reckoned with. This amount includes only the chemical conversion and the heating to the process temperature without any losses. This energy requirement can be slightly influenced through modification and treatment of the raw material mixture.
In practice, the theoretical energy consumption increases significantly in particular due to three additional sources of heat loss: heat losses through the walls of the melting plant, heat losses due to the conducting of the hot combustion gases out of the melting plant, and heat losses due to the heat content of the melt flow itself leaving the melting plant towards the processing. Despite all technological improvements, in the currently existing art a specific energy consumption of more than 4 GJ/t is standard. The ability to influence the named heat sinks with the goal of lowering the energy requirement is possible only within limits. Possible measures are:                The portion of raw material mixture can be replaced by recycled shards, depending on availability, up to more than 90%. This significantly reduces the energy requirement for the chemical conversion of the raw material mixture.        The melt energy requirement can also be reduced by improving the heat conductivity of the raw materials. This is done through pelleting, a solution that however requires a large outlay and is expensive due to the necessary comminution of the raw materials and thermal treatment.        The energy consumption can also be improved to a limited extent by modifying the composition of the components. Working against this, however, are further technical facts that increase the energy consumption.        
Additional energy is required to give a not insignificant volume of the glass melt in the continuous process the dwell time needed in order to remove solid or gaseous relicts. Solid relicts are residues of the less soluble raw material mixture, and gaseous relicts are bubbles that result from the decomposition process of the raw materials. Due to the comparatively high viscosity, even at a high process temperature, this outlay is considerable. Above their melting point, metal melts have the viscosity of water. In such melts, bubbles rise in a short time. In the glass melt, this rise speed is slower by orders of magnitude. The melt vessel therefore has to be made correspondingly large.
The heat losses through the walls of the melting tank mentioned above are proportional to the surface of the glass melting plant. Over the course of decades, the wall design of melting tanks has constantly improved, and in addition new materials have been developed in order to minimize these losses.
An important heat sink is the heat content of the combustion gases. The size of the combustion chamber is designed such that the dwell time of the combustion gases is as long as possible, and the volume or surface of the combustion chamber is kept low due to the wall losses. The heat in the exhaust gas is as a rule used to preheat the combustion air. This heat exchanger process is however physically limited in its effectiveness.
Finally, as a further essential heat sink there remains the glass melt itself, which leaves the melting tank and is conducted through a channel system for processing. The processing temperature is as a rule at least 200° C. below the exit temperature from the melting tank. A correspondingly high cooling power again has the consequence of an unnecessary heat loss. Efforts are therefore made to keep the temperature of the melt flow from the melting tank as low as possible.
All approaches to the improvement of the energetic efficiency have in common the reduction of the size of the melting tank. The evaluation of the energy consumption and melting performance of numerous melting plants shows a significant association. The reduction of the size of the melting tank goes together with a reduction of the wall surface and wall losses, assuming good insulation of the walls. However, the reduction of the size of the melting tank causes a reduction of the quantity of glass that can be produced. A specific load (specific melting performance) of a melting plant of approximately 3.5 t/m2d can be achieved today under particular advantageous technical melt conditions. It is desirable to increase the throughput of a glass melting plant, and correspondingly the specific melting performance, with glass quality that at least remains the same, while here the energy consumption should continue to be kept minimal.
The melt surface is the critical measure for the energetic optimization of a glass melting plant. Conventional melting plants have a melt surface between 30 m2 and 200 m2.
In the melting tank, the still-unmelted raw material mixture coming from the doghouse floats on the melt surface. Here, the coverage should be uniform and spread as thin as possible. The raw material mixture is distinguished, in the still-unmelted state, by very low heat conductivity, and, as a loose heap, has a comparatively high porosity. This further reduces the heat conduction. A heat conductivity of the batch of approximately 1 W/mK is usually assumed. This is lower by more than a factor of 10 compared to the thermal conductivity of the glass melt, differing depending on the color and transmission. The transfer of heat to the batch covering is therefore very limited, and is the reason for seeking to make the covering as thin as possible. All attempts to accelerate the melting of the raw material have in common that the energy density is intensified either over the raw material covering or underneath.
From EP 0 137 881 B1 and U.S. Pat. No. 4,381,934, it is known that the energy input to the raw material stream takes place exclusively from above, via radiation. Here the raw material forms an inclined flow plane from which melting off takes place. In EP 1 904 408 B1, the energy input takes place via burners that are directed onto the melt bath from above. However, these technical teachings have a plurality of disadvantages: the direct burner impulse, in contact with the still-unmelted raw material, causes significant formation of dust. More easily melted components detach from the overall mixture. The melt becomes non-homogenous. The high energy density, caused by the direct burner contact with the raw materials, can cause significant vaporization of the components with high vapor pressure (for example alkalis). Here as well, non-homogeneity of the glass melt is to be expected.
In Glastechn. Ber. 59 (1986) 10, pp. 279-291, Ungan describes the physical limits of the transmission of energy to the unmelted batch. According to this reference, the raw material mixture floating on the glass melt absorbs, approximately in equal portions, the energy via the radiation in the combustion chamber and through heat conduction via the melt from below. The best possible efficiency of the melting off of the raw material mixture can take place only if both energy streams are available. Only this makes it possible to reduce the melt surface necessary for the spreading of the batch. For the energy input underneath the floating batch, it is necessary that in the melting container surfaces are also available that are open and not covered by batch. Only then can the melt absorb the energy and, carried by the density convection, transport the energy under the batch covering. A significant enlargement of the melting tank, necessary for the application of this principle in order to obtain more surface not covered by batch, is however very expensive technically and financially.